A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at a pressure well above one atmosphere (e.g. 8-12 atmospheres). The lamp also includes an electrically or percussively activated primer for igniting the combustible to flash the lamp. During lamp flashing, the glass envelope is subjected to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. Understandably, it is essential to contain the glass envelope in order to protect the consumer who uses the typical photoflash device having at least one and usually several (e.g., 4, 6, 8 or 10) such lamps located therein.
Heretofore, there have been several diverse techniques employed to provide coatings on lamps of the type defined. One approach has been to reinforce the glass envelope by applying a protective coating of cellulose acetate lacquer on the lamp envelope by means of a dip process. In the typical dip process, a large number of envelopes are loaded onto a rack and then sequentially dipped in the cellulose acetate lacquer and oven-dried a sufficient number of times to build up the desired coating thickness. The process is time-consuming, generally requires a large area of production floor space and involves considerable hand labor, all of which add significantly to manufacturing costs. The solvent, generally comprising acetone, is highly flammable and introduces a high risk of fire by ignition of vapors in either the dip bath or drying oven. Injuries to personnel is thus possible, as is the possibility of equipment failure. In addition, consumption of fire extinguishing chemicals resulting from solvent fires further adds to the manufacturing costs.
Another technique utilized in the industry has been to employ a special hardglass material for the envelope, in addition to a protective dip coating. An example of such a glass material is described in U.S. Pat. No. 3,506,385 (K.H. Weber et al). This material, borosilicate, typically consists essentially of the following constituents: 60 to 75 percent by weight SiO.sub.2, 10 to 25 percent by weight B.sub.2 O.sub.3, 1 to 10 percent by weight A1.sub.2 O.sub.3, 4 to 10 percent total alkali oxides (e.g, Na, K, and Li oxides), and 0 to 5 percent by weight BaO. Although providing some degree of improvement in the containment capability of lamp envelopes, the use of dip coatings and hardglass also present signifiant disadvantages in the area of manufacturing cost and safety. More specifically, the hardglass incurs considerable added expense over the more commonly used softglass due to both increased material cost and the need for special lead-in wires (e.g., iron-nickel-cobalt alloy) to provide sealing compatibility with the hardglass material. In addition, even though more resistant to thermal shock, hardglass envelopes can also exhibit cracks and crazes upon lamp flashing, and thus do not obviate the need for a protective coating.
A further procedure has involved the application of photopolymer coatings to the lamp envelope and thereafter curing these coatings by irradiation with a source of ultraviolet (UV) light. In one example, the lamp is held vertically with the base up and dipped into a vat of the photopolymer at 60.degree. C. and extracted very slowly, the dip process taking about 45 seconds. The resulting coating thickness is about 0.020 inch. According to an alternative approach of the method, the flashlamp, while revolving, is sprayed with the liquid photopolymer and then transferred directly into the ultraviolet lamp chamber. Added reinforcement is possible by the use of glass fibers which, for example, may be wrapped about the envelope prior to dipping in the liquid photopolymer, or by premixing short fibers in the photopolymer and applying the coating having said fibers therein. A somewhat critical aspect of the aforementioned UV cured coating is that the shape and uniformity of thickness depends on the flow characteristics of the photopolymer resin as influenced by the force of gravity, orientation of the lamp after coating, and viscosity of the resin. Changes in resin viscosity resulting from changes in temperature affect both the repeatability of the shape of the coating and the uniformity of thickness. These irregularities are retained once the coating is hardened. To overcome this, it has been necessary to design complex manufacturing equipment and to completely re-orient (to a horizontal position) the lamp envelopes during coating. An example of this more recent method, as well as apparatus for applying such a coating, is described in U.S. Pat. No. 4,197,333 (B.H. Leach et al).
Still another approach to providing an improved containing vessel is described in U.S. Pat. No. 3,893,797, (H.L. Hough et al) wherein a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope. The method of applying the coating comprises: placing the glass envelope within a preformed sleeve of the thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the glass envelope; and, simultaneously heating the assembly incrementally along its length, whereby the temperature and vacuum cause the thermoplastic to be incrementally formed onto the glass envelope with the interface substantially free of voids, inclusions and the like. This method provides an optically clear protective coating by means of a significantly faster, safer, and more economical manufacturing process, which may be easily integrated on automated production machinery.
Yet another, substantially earlier, approach to protective coatings for lamps is disclosed in U.S. Pat. No. 3,223,273 (L. Thorington), wherein the exterior surface of the glass envelope of an incandescent lamp is coated with an adhesive resin such as a phenyl or methyl polysiloxane, and then wrapped with a layer of fiber glass yarn having an index of refraction about the same as that of the resin. Prior to wrapping, the yarn is treated with a wetting agent in solvent solution form. Upon completing the dipping and wrapping processes, the covered lamp is placed in a baking oven to cure the resin. The lamp may then be redipped in resin and again cured. Although providing a relatively strong lamp coating, this approach is characterized by many of the same manufacturing disadvantages of the aforementioned solvent dipping process.
In U.S. Pat. No. 3,612,850 (L.M. Nijland et al), there is described a procedure for surrounding portions of individual lamp envelopes in a multilamp device for purposes of hopefully preventing possible lamp explosion. As shown therein, an acrylate resin (e.g., a methyl methacrylate monomer polymerized by the addition of benzoyl peroxide) is located along and substantially about the longitudinal side walls of each envelope after the structure, including all four lamps and corresponding individual reflectors, is arranged in a mold. There are several disadvantages with this technique. Firstly, it fails to assure total lamp containment in view of the apparent necessity for exposing large areas of each lamp's upper and lower regions. Accordingly, it is essential in U.S. Pat. No. 3,612,850 to provide additional safety means such as the illustrated base and outer cover components. It is also apparent from this patent that the apparatus utilized to provide the desired partial containment must be relatively complex in order to assure aligned orientation of the several reflectors and lamps while locating the acrylate resin in place. In view of this requirement, the method does not readily lend itself to mass production at relatively low costs, nor can the method be easily adapted to processing of photoflash devices of different configurations, e.g., the more recently introduced, substantially planar 8 and 10 lamp electrically-activated devices such as described and illustrated in U.S. Pat. Nos. 4,169,281 (B. G. Brower et al) and 4,282,559 (E. G. Audesse et al).